An OLED device includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. Tang et al. described this multilayer OLED device in U.S. Pat. Nos. 4,769,292 and 4,885,211.
Efficient white light producing OLED devices are considered a low cost alternative for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have Commission International d'Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color.
The following patents and publications disclose the preparation of organic OLED devices capable of emitting white light, comprising a hole-transporting layer and an organic luminescent layer, and interposed between a pair of electrodes.
White light producing OLED devices have been reported by J. Shi (U.S. Pat. No. 5,683,823) wherein the luminescent layer includes red and blue light-emitting materials uniformly dispersed in a host emitting material. This device has good electroluminescent characteristics, but the concentration of the red and blue dopants are very small, such as 0.12% and 0.25% of the host material. These concentrations are difficult to control during large-scale manufacturing. Sato et al. in JP 7,142,169 disclose an OLED device, capable of emitting white light, made by placing a blue light-emitting layer next to the hole-transporting layer and followed by a green light-emitting layer having a region containing a red fluorescent layer. Kido et al., in Science, Vol. 267, p. 1332 (1995) and in APL Vol. 64, p. 815 (1994), report a white light producing OLED device. In this device three emitter layers with different carrier transport properties, each emitting blue, green or red light, are used to generate white light. Littman et al. in U.S. Pat. No. 5,405,709 disclose another white emitting device, which is capable of emitting white light in response to hole-electron recombination, and comprises a fluorescent in a visible light range from bluish green to red. Recently, Deshpande et al., in Applied Physics Letters, Vol. 75, p. 888 (1999), published a white OLED device using red, blue, and green luminescent layers separated by a hole blocking layer.
However, these OLED devices require very small amounts of dopant concentrations, making the process difficult to control for large-scale manufacturing. Also, emission color varies due to small changes in the dopant concentration. Full-color devices are made by combining white OLEDs with color filters. However, the color filter transmits only about 30% of the original light. Thus, when the white light is passed through the blue color filter, the blue component is very low in luminance intensity. Due to its low intensity, the blue channel of the R, G, B full-color display is required to operate at much higher current density. This reduces the lifetime of the blue color.